Radar storage tube for indicating moving objects



March 16, 1965 Filed March 2, 1962 E. H- EBERHARDT RADAR STORAGE TUBE FOR INDICATING MOVING OBJECTS 3 Sheets-Sheet l SIGNAL IN 2| BEAM INTENSITY MODULATION CIRCUITS 20 DEI'LECTION l [35 CIRCUITS 3| g U N INVENTOR EDWARD H. EBERHARDT ATTORNEYS E. H. EBERHARDT 3,174,071

March 16, 1965 RADAR STORAGE TUBE FOR INDICATING MOVING OBJECTS 3 Sheets-Sheet 2 Filed March 2, 1962 SIGNAL IN MODULATION CIRCUITS 20 ELIE-1 E D 1 423 :E IE 4: T 53 I90 E s Jg I60 49 50 3s /35 4| /34 58" INVENTOR.

EDWARD H. EBERHARDT 52 57 Wm, @4441 9% ATTORNEYS March 16, 1965 E. H. EBERHARDT RADAR STORAGE TUBE FOR INDICATING MOVING OBJECTS Filed March 2, 1962 BEAM l:

INTENSITY MODULATION cmcuns 2O SIGNAL IN 3 SheetsSheet 3 so 1; l I

=i SIGNAL OUT I NVENTOR EDWARD H. EBERHARDT BY l/m fim duhk ATTORNEYS United States Patent Ofitice 3,174,071 Patented Mar. 16, 1965 3,174,971 RADAR STGRAGE TUBE FOR ENDIATING MOVKNG OBJECTS Edward H. Eberhardt, Fort Wayne, Ind, assignor to International Telephone and Telegraph Corporation, Nutley, Ni, a corporation of Maryland Filed Mar. 2, 1962, Ser. No. 177,938 Claims. (Cl. 315-12) This invention relates generally to the transmission of electrical signals which are modulated to convey useful information, such as those employed in the field of television, radar and related imaging systems, and more particularly to an apparatus and method for cancelling undesired fixed pattern background signals.

A commonly encountered problem in detection, imaging and telecommunication systems is the suppression of unwanted fixed pattern, .i.e., repetitive or stationary background signals With respect to the desired transient signals. In the case of image transmission" systems, such as television and radar, the source of backgorund signals may lie entirely within the pickup unit proper, for example shading signals in an image orthicon, or may lie entirely outside of the pickup device, for example radar return echos from fixed environmental objects. In either case, it is often highly desirable and in fact sometimes necessary to suppress or eliminate such background signals prior to final display or utilization of the signals.

Efforts have been made to remove fixed pattern background signals by clipping or signal mixing circuits. However, while such undesired background signals are repetitive from one frame to the next, they generally are of non-uniform amplitude throughout each frame with the transient signals superimposed thereon, and thus clipping generally results in loss of the desired signals at least in certain areas. Since the undesired signal is repetitive, efforts have been made to obtain cancellation by storing the undesired signal, in the absence of the desired signal, in some sort of storage device capable of retaining and then returning the stored signal information for comparison with the composite signal containing both the back ground signal and the desired signal. Proposals of this type have included electrical and acoustic delay line storage for time delay comparison in radar systems, magnetic drum and tape storage, photographic film replicas, and electrostatic storage methods. While these systems may be practical for modulation methods with storage time cycles less than one millisecond, they are not adaptable to amplitude modulated television systems or to electron images. To the best of the present applicants knowledge, no present system of background cancellation provides the necessary degree of linearity over the requisite Wide dynamic range of operation in writing, storage and readout for satisfactory background cancellation.

It is accordingly an object of my invention to provide an improved system for background cancellation.

Another object of my invention is to provide an improved system for separating and cancelling background signal components from transient signal components of a composite signal which provides the requisite linearity for use in television, radar and imaging systems.

A further object of my invention is to provide an improved background cancellation filter device which can readily be inserted in existing video channels.

Yet another object of my invention is to provide an improved method for background cancellation.

In accordance with the invention, the limitations on linearity in prior storage cancellation proposals are avoided by direct separation of the fixed pattern background signal from the desired signal in a manner analogous to the separation of direct current signals from alternating current signals in a resistance-capacitance coupling network. The invention in its broader aspects provides an' insulator having opposite sides with means for developing a charge pattern on one of the sides which corresponds to the composite signal and means are provided for discharging the one side of the insulator. Means for repetitively scanning the other side of the insulator with an electron beam are provided together with means for collecting electrons emitted from theother side of the insulator're'sponsive to impingement of the beam thereon. By virtue of the development of a charge pattern containing both the repetitive background and new signal information on one side of the insulator, the discharging of the one side of the insulator, and the repetitive scanning of the other side of the insulator by an electron beam, the repetitive part of the signal causes no output signal to appear, and thus the'output signal is responsive only to the new or transitory part of the input signal.

The above-mentioned and other features and objects of this invention and the manner of attaining them will become more apparent and the invention itself will be best understood by reference to the following description of embodiments of the invention taken in conjunction with the accompanying drawings, wherein:

FIG. 1 is a View schematically illustrating one embodiment of my invention;

FIGS. 2A, 2B, 2C and 2D are diagrams useful in explaining my invention;

FIG. 3 is a schematic illustration of another embodiment of my invention;

FIG. 4 is a fragmentary cross-sectional view of the target electrode of FIG. 3;

H6. 5 is a schematic illustration of a further embodiment of my invention; and

PEG. 6 is a schematic illustration of yet another embodiment of my invention.

Referring now to FIG. 1, there is shown an embodiment of the invention wherein a writing electron beam modulated by the input signal describes a raster pattern on one side of a target electrode and a reading electron beam reads the raster oil of the other side of the target electrode, minus the repetitive background component of the signal written onto the target electrode by the writing beam; this type of device is frequently referred to as be ing of the electrical iii-electrical out type.

Here, an evacuated envelope 10 is provided having an enlarged central portion 11 and neck portions 12 and 13 at its opposite ends. Target electrode 14 is positioned in the enlarged portion 11 of envelope It), extending transversely thereacross and takes the form of an extended area, relatively thin sheet of insulating material such as glass or aluminum oxide (A1 0 The insulator sheet 14 is supported and mounted within central portion 11 of envelope It) in any suitable manner (not shown), as is well known to those skilled in the art.

A conventional electron gun 15 including cathode, control grid, accelerating and focusing electrodes, as is Well known to those skilled in the art, referred to as the writing gun, is positioned in neck 12 of the envelope 10 and is arranged to direct an electron beam 16 toward and onto one side of the insulator sheet 14. Conventional vertical and horizontal deflection electrodes 17 and 18 are provided in the neck 12 forwardly of the electron gun 15 and are connected to conventional deflection circuitry 19 in order to cause the electron beam to be scanned in a raster pattern over the insulator sheet 14. Electron gun 15 is coupled to conventional beam intensity modulation circuitry Zil to which signal input circuit 21 is coupled, thereby to modulate the electron beam 16 responsive to the input signal.

A collector electrode, shown here as being a fine mesh metal screen 22, is provided in the enlarged portion 11 of envelope between the insulator sheet 14 and the writing electron gun 15 for collecting electrons emitted from the surface of the insulator sheet 14 responsive to impingement of the electron beam 16 thereon. The collector electrode 22 is coupled to a suitable source 23 of direct current potential, such as 300-volts.

Another electron gun 24, referred to as the flood gun, is provided in the envelope 10 forwardly of the writing electron gun 15, electron gun 24 being arranged to direct a floodbeam 25 of electrons onto the entire surface area of the side of the insulator sheet 14 toward the writing gun 15. Gun 24 is shown as being of the annular or ring type, a suitable configuration being more fully described and illustrated in Patent No. 2,864,920 issued December 9, 1958 to Paul Rudnick and Michael F. Toohig and assigned to the assignee of the present application. The floodgun 24 is coupled to the collector electrode 22 by a suitable source 26 of direct current potential, such as l0-volts.

Another electron gun 27, referred to as the reading gun, is positioned in neck 13 of envelope 10. Gun 27 is provided with suitable cathode, accelerating and focusing electrodes to direct electron beam 28 toward the side of insulator sheet 14 remote from the writing gun 15. Gun 27 is unmodulated so that the beam 23 is a constant current beam. Suitable vertical and horizontal deflection electrodes 29 and 30 are provided forwardly of the reading gun 27 and are connected to conventional deflection circuitry 31 so as to scan the beam 28 in a raster pattern over the side of the insulator sheet 14 which faces the writing gun 27.

Another collector electrode, shown here as being a fine mesh, metal screen, 32, is positioned in the enlarged portion 11 of the envelop 10 between the insulator sheet 14 and the writing gun 27. Collector electrode 32, which collects electrons emitted from the adjacent side of the insulator sheet 14 responsive to impingement of the electron beam 28 thereon, is coupled to a suitable source 33 of direct current potential, such as 300-volts, by load resistor 34. Output circuit 35 is coupled to the collector electrode 32 by a suitable coupling capacitor 35.

Referring now to FIG. 2A, if it be assumed that a fixed charge pattern is established on the writing side 37 of the insulator sheet 14, as shown by the line 38, i.e., a charge pattern which is repeated in identical form during each frame of the scanning by the writing beam 16, after a number of frames of scanning of the reading side 39 of the insulator sheet 14 by the unmodulated, i.e., constant current reading beam 28, a constant or equilibrium charge level will be established on the reading side 39 of the insulator sheet 14, as shown by the line 43; as is well known to those skilled in the art, the equilibrium charge or potential level 40 will be either the potential of the collector 32 or of the cathode of the reading gun 27, depending upon the mode of operation of the system.

Referring now to FIG. 2B, the fixed charge pattern 38 of FIG. 2A is not automatically established by scanning of the writing side 37 of the insulator sheet 14 by the writing beam 16. When the writing beam 16 impinges upon the target, i.e., the writing side 37 of insulator sheet 14, electrons are emitted therefrom; this emission of electrons is referred to as secondary emission and the collector electrode 22 is provided at a suitable potential so that these secondary electrons are attracted to and collected by the collector electrode. Assuming that the electrons of the writing electron beam 16 have a velocity such that the secondary emission ratio is greater than unity, i.e., more electrons are emitted from the writing surface 37 of the insulator sheet 14 than impinge thereon, the writing surface 37 will then be charged more positively at the point of impact because more electrons leave the surface 37 than arrive. With the collector electrode 22 collecting the secondary electrons emitted from the writing surface 37, the incremental charge per scan of the writing beam 16 on the surface 37 initially depends upon the beam current. Since the writing beam 16 is modulated by the input signal while it is caused to scan a raster on the writing surface 37, the charge pattern 38 is initially developed thereon, varying from point to point as shown. The dielectric sheet 14 is in essence a capacitor, as shown schematically at 41, and thus when the charge pattern 38 is initially developed on the writing surface 37 by the beam 16, it is transferred through the capacity of the dielectric sheet 14 to the reading side 39, as indicated by the line 42. As indicated above, however, successive scans of the reading surface 39 by the reading beam 23 will in essence erase the charge pattern 42 to restore the equilibrium charge 4%. On the writing side 37 of the insulator sheet 14, regardless of the fact that the beam 16 is modulated and thus has variable beam current, unless some means are provided for discharging the charge pattern 33 initially formed on the writing surface 37, repeated scans of the surface 37 by the writing beam 16 will produce an equilibrium charge on the writing surface 37, as shown by the dashed line 43. Once an equilibrium charge has been established on the writing surface 37 of the insulator sheet 14, it will not thereafter be changed in any way by any change in the beam current of the wrting beam 16.

Referring now additionally to FIG. 2C, it has been explained that a fixed charge pattern 38 on the writing side 37 of the insulator sheet 14, which would correspond to a fixed background component of the input signal, does not appear on the reading side 39 of the insulator sheet 14 after a number of scans thereof by the reading beam 28. It will be readily apparent that if an equilibrium charge has been established on the reading side 39 of the insulator sheet 14, no output signal will appear in the output circuit 35. However, it has also been explained that the fixed charge pattern 38, while it may initially be established on the writing side 37 by scanning with the modulated writing beam 16, will very quickly disappear and the equilibrium charge level 43 will be established. However, if the charge pattern 38 is wholly or partially discharged during one frame of the scanning by beam 16, it will be established again during the next frame and the writing surface 37 of the insulator sheet 14 will thereby be prevented from reaching the equilibrium charge level 43. In the embodiment of FIG. 1, the writing surface 37 of insulator sheet 14 is continuously discharged by means of the flood electron gun 24. Assuming, as was previously stated, that the writing beam 16 is provided with a velocity so that the secondary emission ratio is greater than unity, then the flood beam 25 provided by the flood electron gun 24 is provided with a velocity such that the secondary emission ratio is less than unity. The flood electrons of the flood electron beam 25 thus tend to cause the writing surface 37 to become more negative, thus tending to discharge the charge pattern 38 established by the writing beam 16.

With the writing surface 37 of the insulator sheet 14 continuously discharged by means of the flood beam 25, the fixed charge pattern 38 corresponding to a fixed background component of the input signal will be reestablished each frame of the scanning by the writing beam 16 and, as hereinbefore explained, will not be reflected on the reading side 39 of the insulator sheet 14- by virtue of the equilibrium charge level 46 established by the reading beam 28. However, assume that a transient signal component is superposed on the background signal component. This transient signal component will result in a variation in the beam current of the writing beam 16 and in turn will establish a transient charge component 44 on the fixed charge pattern 33; the change in charge level is shown in FIG. 2C, as AV. Due to the capacity 41 of the insulator sheet 14, the change AV in the charge level on the writing surface 37 of the insulator sheet 14 will appear in the charge level on the reading surface 39,

at 45. If the charge pattern 44 suddenly appeared and did not subsequently move, successive scans of the reading side 39 of the insulator sheet 14 by the reading beam 28 would again result in the establishment of the equilibrium charge level 40. However, if the transient charge 44 in fact corresponds to a transient or moving input signal component, it will move in succesive scans of the writing surface 37 by the writing beam 16, as shown in the dashed line 46, and thus will successively appear in the charge pattern on the reading side 39, as at 4 7.

The reading beam 28 endeavors to charge the reading surface 39 to a potential near the collector potential, i.e., the equilibrium charge of potential level 40, at which point the secondary emission ratio becomes unity. However, if the charge or potential level on the reading surface 39 is lower or higher than the equilibrium level 40, the secondary emission ratio will increase above unity or decrease below unity, as the case may be; this change in secondary emission is reflected in a change in the current flowing in the circuit coupled to the collector 32 and in turn is reflected in the voltage drop across load resistor 34 and through the coupling capacitor 36 to the output circuit 35.

It will be readily understood that whereas the writing beam 16 has been assumed to have a velocity such that the secondary emission ratio is greater than unity, the system is equally operable with the writing beam 16 having a velocity such that the secondary emission ratio is less than unity, in which case the velocity of the electrons of the flood beam 25 is such that the secondary emission resulting therefrom is greater than unity.

Reference to FIG. 2D in which an equivalent circuit of the system of FIG. 1 is shown will further assist in an understanding of its mode of operation. Here, the capac ity of the insulator sheet 14 in one incremental area between the writing surface 37 and the reading surface 39 is shown at 41, and the capacity from such incremental surface areas to the collector screens 22 and 32, respectively, is shown at 22a and 32a. The resistance of the modulated beam 16 is shown by the variable resistor 16a and the discharge resistance of the floodbeam 25 is shown at 25a. The scanning action of the writing beam 16 which moves the beam onto and oil of the incremental area of writing surface 37 defining one side of capacitor 41 is indicated by a switch 19a and likewise the scanning action of the reading beam 28 which moves the beam onto and off of the incremental area of reading surface 39, which defines the other plate of capacitor 41, is shown by the switch 31a. In the present instance, the resistance of the reading beam 23 may be ignored.

It will now be assumed that the reading beam 28 is being scanned with the same frame rate as the writing beam 16, but slightly lagging with respect thereto. It will further be asumed that the writing beam 16 is impinging upon the incremental area of surface 37 which defines one side of the capacitor 41, which means that switch 19a will be closed and switch 31a open. Under these circumstances, variable resistor 16a will be adjusted to correspond to the instantaneous beam current of the writing beam 16. Assuming that the capacitors 41, 22a and 32a are fully discharged and further assuming the absence of the resistor 25a, i.e., the absence of the flood beam 25, momentary closing of the switch 19:: (corresponding to the scanning of the beam 16 over the incremental area of surface 37 of insulator 14 forming capacitor 41) will result in a momentary flow of current from source 23 through variable resistor 16a and capacitor 22a on the one hand and capacitors 41 and 32a in shunt therewith on the other hand. It will be readily seen that in the absence of the discharge resistor 25a, as soon as the capacitor 22a on the one hand and the capacitors 4i and 32a on the other hand become fully charged either during the first closing of switch 19a or after subsequent closings thereof, no subsequent variation of the variable resistor 16a will result in a change in the potential at point 49, i.e., the incremental area of surface 37 of the insulator 14, which defines one plate of the capacitor 41. Thus it is seen that in the absence of means for discharging the writing surface 37 of the insulator sheet 14, once the equilibrium charge 43 has been established, subsequent variation in the beam current of beam 16 will not result in any change in the charge or potential level on the surface 3'7 of the insulator 14.

it will be seen, however, that with the addition of the discharge resistor 25a in FIG. 2D, when the switch 1% is opened, capacitor 22a on the one hand and 41 and 32a on the other hand will discharge through the resistor 25a, the rate of discharge, of course, depending upon the specific resistance and capacitance values. Thus, even though the capacitors 22a, 41 and 32a were fully charged at the time of opening of the switch 19a at the time of the next closing, they will be at least partially discharged, thus permitting them to be at least partially recharged during the period in which switch 1% is again closed. With the capacitors 23a and 41, 32a being discharged by resistor 25a during the periods in which 19a is opened, the rate of recharging capacitors 22a and 41, 32a during the next period during which 19a is closed will be dependent upon the resistance of variable resistor 16a, i.e., the beam current or" beam 16. Thus, the charge or potential level at point 49 at the instant of opening switch 19a will be dependent upon the resistance of variable resistor 36a. Thus, with the provision of the discharge resistor 25a (the flood beam 25), capacitors 22a and 41, 32a are permitted to recharge to a value something less than saturation or equilibrium, that value being dependent upon the resistance of variable resistor 16a.

Assuming now that switch 1% has ben opened, and switch 31a is closed. It will be recalled that capacitors 41 and 32a have previously been charged and at this instant are in the process of discharging through the discharge resistor 25a so that point 49 has at that instant a finite charge or potential. To the extent that the charge or potential at point 49 at the instant of closing of the switch 31a is above or below the potential established by the source 33, current will flow to charge capacitors 22a and 41 on the one hand and 32a on the other hand in such a direction to cause the charge or potential level at point 5% to equal the potential of source 33. Now assuming the presence of a fixed background component on the input signal so that the beam current of writing beam 16 has the same value at a given location on the writing side 37 of the insulator sheet 14 during each scan, i.e., variable resistor 16a has the same setting each time switch 1% is closed, and further assuming that the switch 31a is closed the same interval after each opening of switch 19a, i.e., reading beam 28 being scanned with the same frame rate as the writing beam 16, the charges on capacitors 22a and 41, on the one hand, and 32a on the other hand caused by current flowing when switch 31a is closed and the charge or potential level at point 56 is not equal to the potential of source 33, will continuously accumulate so long as the potentials are not equal with the net effect that the charge or potential level at point 54) will eventually reach that of source 33. Thus, it is seen that by virtue of the discharge path provided by the flood beam 25, a fixed charge or potential pattern 38 may be established on the writing surface 3'7 of the insulator sheet 14 in response to a fixed or background signal component of the input signal, and that such fixed charge pattern is not transferred to the reading surface 59 of the insulator sheet 14, the reading beam 28 on the contrary after a number of scans developing a fixed charge or potential pattern 40.

Assuming now that a transient signal component appears in the input signal which results in the beam 16 Writing the transient charge pattern 44 on the writing surface 37 of the insulator sheet 14, this has the same effect as a variation in the setting of the variable resistor 16a at the instant the switch 19a is closed, thus resulting in recharging of the capacitors 22a on the one hand and 41, 52:: on the other hand to a difli'erent level and the development of a dilferent charge or potential level at point 49 at the instant the switch 19a is opened. Thus, a different charge or potential level will appear at point which is represented at 45 in FIG. 20. As previously indicated, when the switch 31a is closed, capacitors 22a and 41 on the one hand and 32:: on the other hand will be charged in a direction to restorepoint 5% to the potential level of source 33, this change in current being reflected through the coupling capacitor 36 to the output terminal 35.

It will be readily comprehended that the scanning of the reading surface 35 of the insuatorsheet 14 by the reading beam 28 is preferaby at the same frame rate as the scanning by the writing beam lb, or an integral multiple thereof.

Referring now to FIG. 3, in which like elements are shown by like reference numerals, the means to discharge the charge pattern on the writing surface of the insulator sheet might be in the form of a resistive coating on the writing surface of the sheet rather than the flood gun of the embodiment of PEG. 1. Here, tube 51 again has an evacuated envelope 10 with an enlarged center portion 11 and elongated neck portions 12 and 13. Writing gun 15 is disposed in neck portion 12 and is arranged to direct electron beam 16 toward target electrode 52. Horizontal and vertical deflection elements 17 and 18 are'again provided coupled to suitable deflection circuitry 19 for scanning the writing beam 16 in raster fashion over the surface of the target electrode 52. Beam intensity modulation circuits 20 are coupled to the writing gun 15 with signal input circuit 21 being in turn coupled thereto'for modulating the beam current of the beam 16 in response to the input signal. Collector screen 22 is positioned in enlarged envelope portion 11 spaced from the writing side of the target electrode 52 and is connected to a suitable source 23 of direct current potential, such as 300-vo1ts.

Reading electron gun 27 is positioned in neck portion 13 of the envelope 1i and is arranegd to'direct constant current reading beam 28 onto the reading side of the target electrode 52. Deflection elements 2? and 30 which are coupled to conventional deflection circuitry 31 are provided for deflecting the reading beam 23 over the reading surface of the target electrode 52 in a raster pattern. Collector electrode 32 is provided spaced from the reading surface of the target electrode 52 and is coupled to a suitable source 33 of direct current potential, such as 300-volts by load resistor 34-. Coupling capacitor 36 again connects output terminal to the collector electrode 32.

Referring now in addition to FIG. 4, the target electrode 52 of the embodiment of FIG. 3, comprises an extended area, relatively thin sheet 53 of insulator material having a fine mesh metal screen 54 arranged abutting the writing surface 55. In actual practice, the screen 54 may serve as a support for the insulator sheet 53, which may be a film of suitable insulator material such as aluminum oxide (A1 0 The fine mesh metal screen 54 and the writing surface 55 of the insulator sheet 53 in the interstices of the screen 54 is coated with a relatively thin layer 56 of resistive material, such as porous antimony trisulflde (Sb S The equivalent circuit of this arrangement is shown in FIG. 4 in which the incremental capacitance of the insulator sheet 53 between the writing surface 55 and the reading surface 57 is shown as capacitor 41, the leakage resistance between the resistive layer 56 and the fine mesh metal screen 54 is shown as resistor 58, and the shunt capacitance between the resistive layer 56 and the fine mesh metal screen 54 is shown as capacitor 59. The fine mesh metal screen 54 is connected to the collector electrode 22 by a source 60 of direct current potential, such as minus lO-volts. It will be seen that the resistive coating 55 performs the same function as the flood beam 25 of the embodiment of FIG. 1 and the resistor 25:: of the equivalent circuit of FIG. 21).

While in the embodiment of FIG. 3 the writing beam 16 is modulated, it will be readily apparent that the writing beam may be unmodulated and the modulation a plied to the circuit or" the resistive coating 56.

Referring now to FIG. 5, in Which'like elements again are indicated by like reference numerals, my invention is equally applicable to a device in which the input is in the form of a radiation image rather than in the formof an amplitude modulated electrical signal, as in the case of the embodiments of the previous figures. Here, the tube 61 has an enlarged portion 62 having its end 63 transparent to the frequency spectrum ofthe radiation image being received. Tube 61 has a neck portion 64 in which reading electron gun 27 is disposed arranged to direct constant current electron beam 28 toward target electrode 55. Beam 28 is deflected in raster fashion over the reading surface of the target electrode 65 by deflection elements 29 and 33 coupled to conventional deflection circuitry 31. Collection electrode 32 is provided spaced from the reading surface 66 of the target electrode 65 and connected to a suitable source 33 of direct current potential, such as 300volts, by load resistor Output circuit 35 is again coupled to collector electrode 32 by coupling capacitor 36.

The target electrode 65 comprises an extended area of relatively thin sheet 67 of insulator material having a fine mesh metal screen 68 abutting its writing surface 69, The line mesh metal screen 68 is coupled to 'a suitable source 79 of direct current potential, such as l0-volts. The fine mesh metal screen 68 and the writing surface 69 of the insulator sheet 67 in the interstices of the screen 68 is coated with a layer 71 of suitable material which has its conductivity varied'in response to incident radiation. Thus, the layer 71 may be a suitable photoconductor which has its resistance varied in response to incident light, either in the visible spectrum or in the X-ray or infra-red regions, or the layer 69 may be the type which has its resistance variable in, response to temperature. In any event, a suitable radiation image 72 is focused by a suitable lens system 73 through the end 63 of the tube 61 to form a radiation image 74 on the radiation-sensitive layer 71. Focusing of the radiation image 74 upon the radiation-sensitive layer 71 will produce a charge or potential pattern thereon in much the same manner as the writing beam 16 of the embodiments of the previous figures.

In order to discharge the charge or potential pattern formed on the writing surface 69 of the insulator sheet 67, flood electron gun 24 is provided arranged to direct a flood beam 25 onto the radiation-sensitive coating 71 and the charge or potential pattern thereon responsive to the radiation image 74 will thus be discharged as previously described. It will be readily understood that the operation of the tube 61 on the reading side of insulator sheet 67 is identical to that of the embodiments of the previous figures.

Referring now to FIGURE 6 in which like elements are still indicated by like reference numerals, it will be readily apparent that the means for discharging the charge or potential pattern on the writing side of the insulator sheet need not continuously discharge the writing surface, but on the contrary need only provide an average discharge. Thus, in the embodiment of FIG. 6, the writing beam itself is pulsed alternatively to provide a high velocity beam for writing the charge pattern onto the writing side of the dielectric sheet and a low velocity beam for discharging purposes. Here, the tube 75 has envelope 10 with enlarged portion 11 and neck portions 12 and 13. Writing gun 76 is positioned in neck portion 12 and is arranged to direct electron beam 77 toward the Writin g surface 37 of the extended area insulator sheet 14. Deflection elements 17 and 18 are arranged to deflect the beam 77 in raster fashion over the writing surii face 37 of the insulator sheet 14 and are connected to suitable deflection circuitry 19. Collector electrode 78 is spaced from the writing surface 37 of the insulator sheet 14.

Reading electron gun 27 is disposed in neck portion 13 of envelope 1t and is arranged to direct unmodulated electron beam 28 onto the reading surface 39 of the insulator sheet 14. Beam 28 is deflected in raster fashion by deflection elements 29 and 3t) which are coupled to conventional deflection circuitry 31. Collector electrode 32 is again coupled to a suitable source 33 of direct current potential, such as 300-volts, by load resistor with output circuit 35 being connected to the collector electrode 32 by coupling capacitor 3%.

In this embodiment, for Writing the charge or potential pattern onto the writing surface 37 of the insulator sheet 14, the Writing electron gun '76 is coupled by switch 79 to conventional beam intensity modulation circuitry to which the signal input terminal 21 is coupled. With switch 79 coupling the writing gun 7% to the beam intensity modulation circuitry Zti, the beam '77 is provided with high velocity and is modulated in "espouse to the input signal. During Writing of the charge pattern on the writing surface 37 of the insulator sheet 14, the collector electrode '78 is connected to a suitable source 8d of direct current potential, such as tl-volts, by switch 81 which is ganged with the switch 79. During reading, switch 79 couples the gun 7-5 to another source 82 of direct current potential, such as G-volts, and switch 31 couples the collector electrode '13 to another source 33 of potential, such as lO-volts, which is in series opposition with the source 86 as shown.

In the embodiment of the invention as shown in FIG. 6, the tube 75 need have neither a resistive layer on the target electrode nor a separate flood gun, discharging of the charge pattern on the Writing side 37 of the insulator sheet 14 being provided by the electron beam 77 during its low velocity. In this embodiment, the scan by the reading beam 28 of the reading surface 39 of the insulator sheet 14' can be arranged to occur before the stabilization or discharging scan by the beam 77 in its low velocity period, and therefore, before there has been any substantial reduction in the char e or potential level on the writing side of the insulator sheet due to leakage. Direct synchronization or phasing of the writing and reading scans is not necessary with this embodiment so long as the reading scan is arranged to occur only in intervals between writing with the high velocity bearand stabilization or discharging with the low velocity beam.

It will also be understood that the two functions described above or" writing a signal onto side 37 of insulator sheet 14 with a high velocity beam and subsequently partially discharging side 37 with the same beam in a low velocity mode may be accomplished in other ways. For example, the gun 76 may be operated in such a way as to provide a high velocity beam for both functions, i.e., writing and discharging, with the collector 7S switched by switch 81 between positive and negative potentials in order to write positively and discharge negatively, or between negative and positive potentials in order to write negatively and discharge positively, as required. As another example, the signal may be written onto side 37 of dielectric sheet l twith a low velocity beam and subsequently partially discharged wtih the same beam operated in a high velocity mode.

It will now be readily seen that the electrical input tubes of FIGS. 1, 3 and 6 may be inserted in an amplifier chain at a relatively high level so that any beam noise which is generated in the tube will be harmless.

While I have described above the principles of my invcntion in connection with specific apparatus, it is to be clearly understood that this description is made only by id way of example and not as a limitation to the scope of my invention.

What is claimed is:

1. A device for separating and cancelling a repetitive background signal component from transient signal components of a composite signal comprising: an insulator having opposite sides; means for developing a stored charge pattern on one of said sides of said insulator corresponding to said composite signal; means for discharging said one side of said insulator to prevent an equilibrium charge thereon and permit coupling of said transient signal to the other side; means for repetitively scanning the other of said sides of said insulator with a constant electron beam to establish an equilibrium charge; and means for collecting electrons emitted from said other side of said insulator responsive to impingement of said beam and transient signal thereon.

2. A device for separating and cancelling a repetitive background signal component from transient signal components of a composite signal comprising: an evacuated envelope; a relatively thin sheet of insulating material in said envelope; means for developing a stored charge pattern on one side of said sheet corresponding to said composite signal; means for discharging said one side of said sheet to prevent an equilibrium charge thereon and permit coupling of transient signals to the other side; electron gun means in said envelope for directing an electron beam having constant beam current toward the other side of said sheet to establish an equilibrium charge; means for repetitively scanning said beam over said other side of said sheet; means in said envelope for collecting electrons emitted from said other side of said sheet responsive to impingement of said beam and transient signal thereon; and an output circuit coupled to said collecting means.

3. The combination of claim 2 wherein said charge pattern developing means comprises another electron gun means in said envelope for directing another electron beam toward said one side of said sheet, means for modulating said other beam responsive to said composite signal, means for repetitively scanning said other beam over said one side of said sheet, and other means for collecting electrons emitted from said one side of said sheet reponsive to impingement or" said other beam thereon.

4. The combination of claim 2 wherein said discharging means comprises another gun means in said envelope for directing a flood beam of electrons onto the entire area of said one side of said sheet, and means for collectins electrons emitted from said one side of said sheet responsive to impingement of said flood beam of electrons thereon.

5. The combination of claim 2 wherein said charge pattern developing means comprises a conductive grid abutting said one side of said sheet, a layer of material having its conductivity varied in response to incident radiation coating said grid and said one side of said sheet in the interstices of said grid, and means for directing a radiation image onto said layer; and wherein said discharging means comprises another electron gun means in said envelope for directing a flood beam of electrons onto the entire area of said layer.

6. The combination of claim 2 further comprising another electron gun means in said envelope for directing another electron beam toward said one side of said sheet, means for repetitively scanning said other beam over said one side of said sheet, and other means for collecting electrons emitted from said one side of said sheet responsive to impingement of said other beam thereon; and wherein said charge pattern developing means comprises means for at times providing said other beam with high velocity and modulating the same responsive to said composite signal; and wherein said discharging means comprises means for at other times providing said other beam with inverse charging and constant beam current.

7. A device for separating and cancelling a repetitive background signal component from transient signal components of a composite electrical input signal comprising: an evacuated envelope; a relatively thin extended area sheet of insulating material in said envelope; a first electron gun in said envelope directing a first beam of electrons toward one side of said sheet; means for deflecting said first beam repetitively to scan said one side of said sheet; an input circuit for receiving said input signal; a source of potential and means coupling the same and said input circuit to said first electron gun for modulating the current of said first electron beam responsive to said composite signal; a first collector electrode in said envelope spaced from said one side of said sheet; a source of potential and circuit means coupling the same to said first collector electrode so that said first collector electrode collects electrons emitted from said one side of said sheet responsive to impingement of said first beam thereon whereby said first beam develops a charge pattern on said one side of said sheet having a fixed component corresponding to said background signal component and a transient component corresponding to said transient signal component; means for continuously discharging said one side of said sheet thereby preventing establishment of an equilibrium charge on said one side of said sheet and permitting coupling of said transient signal to the other side; a second electron gun in said envelope directing a second beam of electrons toward the other side of said sheet, a source of potential and circuit means coupling said second electron gun thereto whereby said second beam has constant current; means for deflecting said second beam repetitively to scan said other side of said sheet; a second collector electrode in said envelope spaced from said other side of said sheet; a source of potential and circuit means coupling the same to said second collector electrode so that said second collector electrode collects electrons emitted from said other side of said sheet responsive to impingement of said second beam thereon whereby said second beam develops an equilibrium charge on said other side of said sheet and causes electron emission therefrom varying in accordance with said transient signal; and an output circuit coupled to said second collector electrode for providing an output signal responsive to the change in current in said lastnamed circuit means due to said transient signal.

8. The combination of claim 7 wherein said means for deflecting said first beam has a predetermined frame rate, and wherein said discharging means discharges said one side of said sheet at a rate slower than said frame rate.

9. The combination of claim 7 wherein said means for deflecting said first beam has a predetermined frame rate, and wherein said means for deflecting said second beam has a frame rate which is an integral multiple of said predetermined frame rate.

10. The combination of claim 7 wherein said means for deflecting said first beam has a predetermined frame rate, and wherein said means for deflecting said second beam has the same frame rate.

11. The combination of claim 7 wherein said first electron gun provides said first electron beam with a velocity to cause secondary emission of electrons from said one side of said sheet with a ratio of one sign with respect to unity; and wherein said discharging means comprises a third electron gun in said envelope for directing a flood beam of electrons toward the entire area of said one side of said sheet, and another source of potential and circuit connections coupling the same to said third electron gun whereby said flood beam has constant current and a velocity to cause secondary emission of electrons from said one side of said sheet with a ratio of the opposite sign with respect to unity.

12. A device for separating and cancelling a fixed component from transient components of a composite radiation image comprising: an evacuated envelope; a relatively thin extended area sheet of insulating material in said envelope; a fine mesh metal screen abutting one side of said sheet; a relatively thin la er of material having its conductivity varied in response to incident radiation coated on said screen and said one side of said sheet in the interstices of said screen; means for focusing a radiation image on said coating; a source of potential and circuit connections coupling the same to said screen Whereby said image on said coating develops a charge pattern on said one side of said sheet having a fixed component corresponding to said fixed component of said image and transient components corresponding to said transient components of said image; a first electron gun in said envelope directing a flood beam of electrons toward the entire area of said screen on said one side; a source of potential and circuit connections coupling the same to said electron gun so that said flood beam has constant current whereby said one side of said sheet is continuously discharged thereby preventing establishment of an equilibrium charge on said one side of said sheet and permitting coupling of said transient components to the other side; a second electron gun in said envelope directing a second beam of electrons toward the other side of said sheet; a source of potential and circuit means coupling said second electron gun thereto whereby said second beam has constant current; means for deflecting said second beam repetitively to scan said other side of said sheet; a collector electrode in said envelope spaced from said other side of said sheet; a source of potential and circuit means coupling the same to said collector electrode so that said collector electrode collects electrons emitted from said other side of said sheet responsive to impingement of said second beam thereon whereby said second beam develops an equilibrium charge on said other side of said sheet and causes electron emission therefrom varying in accordance with said transient components; and an output circuit coupled to said second collector electrode for providing an output signal responsive to the change in current in said last-named circuit means due to said transient components.

13. A device for separating and cancelling a repetitive background signal component from transient signal components of a composite electrical input signal comprising: an evacuated envelope; a relatively thin extended area sheet of insulating material in said envelope; a first electron gun in said envelope for directing a first beam of electrons toward one side of said sheet; means for deflecting said first beam repetitively to scan said one side of said sheet; an input circuit for receiving said input signal; .a first source of potential and beam intensity modulation means coupled thereto and to said input circuit for modulating the current of said first electron beam responsive 'to said composite signal; a second source of potential; a first collector electrode in said envelope and spaced from said one side of said sheet; third and fourth sources of potentral; first switching means for alternately coupling :satd beam intensity modulation means and said second source of potential to said first electron gun; second switching means synchronized with said first switching means for alternately coupling said third and fourth sources of potential to said first collector electrode whereby said first collector electrode collects electrons emitted from said one side of said sheet responsive to impingement of said beam thereon; said first and third sources of potential cooperating to provide said first electron beam with a velocity to cause secondary emission of electrons from said one side of said sheet with a ratio of one sign with respect to unity whereby said first beam a on 7 ng a fixed component correspondmg to said background signal component and a transient component corresponding to said transient signal component; said second and fourth sources of potential cooperating to provide said first beam with constant beam current and a velocity to cause secondary emission of electrons from said one side of said sheet with a ratio of the opposite sign with respect to unity whereby said one side of said sheet is discharged thereby preventing establishment of an equilibrium charge thereon; a second electron gun in said envelope for directing a second beam of electrons toward the other side of said sheet; a source of potential and circuit means coupling said second electron gun thereto whereby said second beam has constant current; means for deflecting said second beam repetitively to scan said other side of said sheet; a second collector electrode in said envelope spaced from said other side of said sheet; a source of potential and circuit means coupling the same to said second collector electrode so that said second collector electrode collects electrons emitted from said other side of said sheet responsive to impingement of said second beam thereon whereby said second beam develops an equilibrium charge on said other side of said sheet in the absence of a transitory charge component on said one side of said sheet and a transitory charge on said other side of said sheet in response to a transitory charge component on said one side of said sheet; and an output circuit coupled to said second collector electrode for providing an output signal responsive to the change in current in said last-named circuit means due to said transitory charge.

14. A device for separating and cancelling a repetitive background signal component from transient signal components of a composite signal comprising: an evacuated envelope; a relatively thin sheet of insulating material in said envelope; means for developing a charge pattern on one side of said sheet corresponding to said composite signal; means for discharging said one side of said sheet, said discharging means comprises a conductive grid abutting said one side of said sheet, and a layer of resistive material coating said grid and said one side of said sheet in the interstices of said grid; electron gun means in said envelope for directing an electron beam having constant beam current toward the other side of said sheet; means for repetitively scanning said beam over said other side of said sheet; means in said envelope for collecting electrons emitted from said other side of said sheet responsive to impingement of said beam thereon; and an output circuit coupled to said collecting means.

15. A device for separating and cancelling a repetitive background signal component from transient signal components of a composite electrical input signal comprising: an evacuated envelope; a relatively thin extended area sheet of insulating material in said envelope; a first electron gun in said envelope for directing a first beam of electrons toward one side of said sheet; means for deflecting said first beam repetitively to scan one side of said sheet; an input circuit for receiving said input signal;

a source of potential and means coupling the same and said input circuit to said first electron gun for modulating the current of said first electron beam responsive to said composite signal; a first collector electrode in said envelope spaced from said one side of said sheet; a source of potential and circuit means coupling the same to said rst collector electrode so that said first collector electrode collects electrons emitted from said one side of said sheet responsive to impingement of said first beam thereon whereby said first beam develops a charge pattern on said one side of said sheet having a fixed component corresponding to said background signal component and a transient component corresponding to said transient signal component; means for continuously discharging said one side of said sheet thereby preventing establishment of an equilibrium charge on said one side of said sheet; a second electron gun in said envelope for directing a second beam of electrons toward the other side of said sheet, a source of potential and circuit means coupling said second electron gun thereto whereby said second beam has constant current; means for deflecting said second beam repetitively to scan said other side of said sheet; a second collector electrode in said envelope spaced from said other side of said sheet; a source of potential and circuit means coupling the same to said second collector electrode so that said second collector electrode collects electrons emitted from said other side of said sheet responsive to impingement of Said second beam thereon wherby said second beam develops an equilibrium charge on said other side of said sheet in the absence of a transitory charge component on said one side of said sheet, and a transitory charge on said other side of said sheet in response to a transitory charge component on said one side of said sheet; and an output circuit coupled to said second collector electrode for providing an output signal responsive to the change in current in said last-named circuit means due to said transitory charge, said discharging means comprising a fine mesh metal screen abutting said one side of said sheet, a relatively thin layer of resistive material coating said screen and said one side of said sheet in the interstices of said screen, and a source of potential and circuit connections coupling the same to said screen whereby said one side of said sheet is continuously discharged.

Reterences Cited by the Examiner UNITED STATES PATENTS 2,687,492 8/54 Szegho et al. 315-13 XR 2,866,918 12/58 Hansen 31511 DAVID REDINBAUGH, Primary Examiner. 

1. A DEVICE FOR SEPARATION AND CANCELLING A REPETITIVE BACKGROUND SIGNAL COMPONENT FROM TRANSIENT SIGNAL COMPONENTS OF A COMPOSITE SIGNAL COMPRISING: AN INSULATOR HAVING OPPOSITE SIDES; MEANS FOR DEVELOPING A STORED CHARGE PATTERN ON ONE OF SAID SIDES OF SAID INSULATOR CORRESPONDING TO SAID COMPOSITE SIGNAL; MEANS FOR DISCHARGING SAID ONE SIDE OF SAID INSULATOR TO PREVENT AN EQUILIBRIUM CHARGE THEREON AND PERMIT COUPLING OF SAID TRANSIENT SIGNAL TO THE OTHER SIDE; MEANS FOR REPETITIVELY SCANNING THE OTHER OF SAID SIDE; MEANS FOR REPETITIVELY CONSTANT ELECTRON BEAM TO ESTABLISH AN EQUILIBRIUM CHARGE; AND MEANS FOR COLLECTING ELECTRONS EMITTED FROM SAID OTHER SIDE OF SAID INSULATOR RESPONSIVE TO IMPINGEMENT OF SAID BEAM AND TRANSIENT SIGNAL THEREON. 